This invention pertains to an anode element for monopolar electrolysis cells arranged in a filter-press type configuration, and especially to those filter-press type cells operated according to the diaphragm process.
Electrolysis cells of this type are used primarily for chlor-alkali electrolysis, which comprises the preparation of chlorine, hydrogen, and alkali hydroxides from aqueous alkali chloride solutions by electrochemical action. Chlorine is also obtained as a by-product of the electrolysis of molten salts used in the manufacture of alkali metals or alkaline earth metals. Cells of this type have also been increasingly used in the electrolytic decomposition of hydrochloric acid, and are becoming more significant in this respect.
Some of these products are produced in very large quantities as basic chemicals. In the case of chlor-alkali electrolysis, plants are frequently operated with individual process loop production capacities of 500 to 1000 tons of chlorine per day. In such plants, current intensities of up to about 500,000 amps are attained. Depending upon the particular process used, larger or smaller numbers of electrolysis cells may be combined into a single circuit.
If an electrical direct current is caused to flow through an electrochemical cell having an alkali chloride-containing aqueous electrolyte, chlorine gas is primarily formed at the positive pole or anode, while hydrogen gas and alkali hydroxide are formed at the negative pole or cathode. Reverse reaction due to mixing of the products should, of course, be prevented. For this purpose, at first two different processes were initially developed: the so-called mercury process and the diaphragm process.
When the diaphragm process is employed in chlor-alkali electrolysis, alkali chloride solution is typically fed into the anode chamber and chlorine is removed at the anode. The alkali ions, together with the remaining depleted alkali chloride solution, migrate through the diaphragm into the cathode chamber. There, the alkali ions are discharged at the cathode where alkali hydroxide and hydrogen form in the presence of water. Thus, a mixture of alkali chloride and alkali hydroxide forms, the so-called cell liquor, which is further processed in order to obtain purified hydroxide. The diaphragm, serving as a porous separating wall, separates the anode chamber from the cathode chamber and thus prevents mixing and the undesirable reverse reaction of the products separated at the electrodes.
In order to provide continuous electrolysis, a uniform liquid flow through the diaphragm into the cathode chamber should be maintained. For this purpose, various liquid levels are maintained in commercial diaphragm cells and thus different hydrostatic pressures are produced in the anode chamber and in the cathode chamber. Since the flow resistance of the diaphragm changes during the operating cycle, for example due to clogging and other similar problems, diaphragm cells currently in use are generally equipped with a characteristic high cover in which a relatively large pressure differential can be maintained. These diaphragm cells are typically designed in the shape of a trough, wherein the cathodes project like fingers into a collar. If a common anode chamber is employed for all anodes then, in the case of chlor-alkali electrolysis, chlorine gas produced at the anodes is gathered together into the high cover mentioned above.
A third electrolysis process, the so-called membrane cell process, has been increasingly used in recent times. Since dimensionally stable anodes and permselective membranes are now available, electrolysis cells can be produced with a thin separating membrane clamped between flat opposed electrodes.
A cell block having a filter-press type configuration can be obtained by joining together several individual electrolysis cells having electrode elements and partitions, such as diaphragms or permselective membranes, located between them. The filter-press type electrolysis cells are well known as shown, for example, in German Pat. No. 1,054,430 and German Offenlegungsschrift No. 2,222,637, the disclosures of which are incorporated herein by reference, illustrating the electrolysis of aqueous hydrochloric acid, and in German Offenlegungsschrift No. 2,510,396, directed to chlor-alkali electrolysis, the disclosure of which is also hereby incorporated by reference.
In general, the cell elements are held in supporting frames. With the aid of a suitable pressing device, for example a hydraulic device, a tension bar, or individual screws, the cell block is pressed together with gaskets placed between the cell elements to seal them off from one another, and subsequently mounted, if desired, on a suitable frame to form a rigid unit, which may have from about 10 up to, for example, 100 cell elements and a corresponding production capacity.
The electrolysis filter-press type cells can then be connected in bipolar or monopolar fashion in accordance with procedures such as illustrated in U.S. Pat. No. 4,056,458, the disclosure of which is hereby incorporated by reference. The monopolar electrode elements generally comprise two parallel electrode surfaces between which the cathode chamber or the anode chamber is formed, depending on the electrical connection of the electrode element. Such an arrangement is illustrated in applicant's concurrently filed patent application No. 039,997 relating to an electrolysis cell system, the disclosure of which is hereby incorporated by reference. Irrespective of the particular design employed, however, it has been found difficult to form different liquid levels without substantial loss of active electrode and diaphragm surface area.
It is thus a primary object of this invention to provide an improved anode element suitable for use in filter-press type electrolysis cells, and particularly suited for use in diaphragm cells, which will provide for improved operating efficiency.